1. Field of the Invention
The present invention relates to a liquid crystal display device used in a personal computer display, a television, a projector, or the like. In particular, the present invention relates to a liquid crystal display device which is excellent in response characteristics and suitable for displaying a moving picture.
2. Description of the Prior Art
Liquid crystal display devices have the advantages that they are thin and light and that they can be driven at low voltages and have low power consumption. Accordingly, liquid crystal display devices are being used in various kinds of electronic devices. In particular, active matrix liquid crystal display devices, in which a thin film transistor (TFT) as a switching element is provided for each picture element, are as excellent even in display quality as cathode-ray tube (CRT) displays. Accordingly, active matrix liquid crystal display devices are being used not only in personal computer displays but also in televisions, projectors, and the like.
A general liquid crystal display device has a structure in which a liquid crystal is contained between two substrates placed to face each other. On one substrate, TFTs, picture element electrodes, and the like are formed. On the other substrate, color filters, a common electrode, and the like are formed. Hereinafter, the substrate having the TFTs, the picture element electrodes, and the like formed thereon is referred to as a TFT substrate, and the substrate placed to face the TFT substrate is referred to as a counter substrate. Furthermore, a structure constructed by filling the space between the TFT and counter substrates with the liquid crystal is referred to as a liquid crystal panel.
On both sides of the liquid crystal panel in the thickness direction, polarizing plates are respectively placed. The amount of light passing through these two polarizing plates can be adjusted by applying a voltage between the picture element electrode and the common electrode to change the state of the alignment of liquid crystal molecules.
Heretofore, twisted nematic (TN) liquid crystal display devices have been widely used in which a liquid crystal with positive dielectric anisotropy is contained between two substrates and in which liquid crystal molecules are aligned in a twisted manner. However, TN liquid crystal display devices have the disadvantage that viewing angle characteristics are poor and that contrast and color greatly change when a screen is viewed from an oblique direction. Accordingly, multi-domain vertical alignment (MVA) liquid crystal display devices having favorable viewing angle characteristics are being developed and commercialized.
In an MVA liquid crystal display device, a liquid crystal with negative dielectric anisotropy is contained between two substrates, and alignment control structures are provided so that a plurality of regions (domains) in which the alignment directions of liquid crystal molecules are different from each other are formed in each picture element when a voltage is applied. As the alignment control structures, for example, protrusions made of dielectric material or slits provided in electrodes are used. Japanese Unexamined Patent Publications No. 2003-195328 and 2003-330043 disclose examples of liquid crystal display devices in which slits provided in picture element electrodes are used as alignment control structures.
FIG. 1 is an equivalent circuit diagram for one picture element of a liquid crystal display device. As shown in this FIG. 1, each picture element of the liquid crystal display device includes a TFT 10, a liquid crystal cell CLC, and an auxiliary capacitance Cs. The liquid crystal cell CLC includes a picture element electrode, a common electrode, and a liquid crystal placed therebetween.
The TFT 10 is turned on or off by a scan signal supplied to a gate bus line 11. When the TFT 10 is turned on, a display signal (display voltage) is supplied from a data bus line 12 to the liquid crystal cell CLC and the auxiliary capacitance Cs. Thereafter, even when the TFT 10 is turned off, the voltage held by the liquid crystal cell CLC and the auxiliary capacitance Cs is applied to the liquid crystal.
In the liquid crystal display device, it takes a long time for all liquid crystal molecules in the picture element to be aligned in a predetermined direction according to a voltage since the application of the voltage between the picture element electrode and the common electrode. Furthermore, the liquid crystal molecules have dielectric anisotropy, and therefore the capacitance value of the liquid crystal cell CLC changes during a period after the application of the voltage before all the liquid crystal molecules are aligned in a predetermined direction. As a result, the voltage applied to the liquid crystal decreases. For this reason, the auxiliary capacitance Cs is connected in parallel to the liquid crystal cell CLC as shown in FIG. 1. Thus, a change in the voltage applied to the liquid crystal is reduced.
However, known liquid crystal display devices have the problem that response characteristics are not sufficient and that a lag occurs when a moving picture is displayed. FIG. 2 is a view showing response characteristics of a known liquid crystal display device with time after the first application of a display signal on the horizontal axis and transmittance (brightness) on the vertical axis. As shown in this FIG. 2, in many known liquid crystal display devices, when a black display state changes into a white display state, desired transmittance is not reached by the first application of a display signal, but the desired transmittance is reached by the second application of a display signal. Generally, when transmittance in a white display is assumed to be 100%, response time is defined using the time (rise time) τr required for the transmittance to change from 10% to 90% and the time (fall time) τf required for the transmittance to change from 90% to 10%.
In order to improve response characteristics of a liquid crystal display device, it is possible to conceive of modifying a liquid crystal material. However, under present circumstances, a liquid crystal material is not obtained which shows sufficient response characteristics when used in a liquid crystal display device and which satisfies both of display performance and long-term reliability.
It is also possible to conceive of increasing the capacitance value of an auxiliary capacitance Cs to reduce a decrease in an applied voltage caused by the dielectric anisotropy of liquid crystal molecules. However, in general, an electrode partially constituting the auxiliary capacitance Cs is formed of metal. Accordingly, if the size of the electrode is increased in order to increase the capacitance value of the auxiliary capacitance Cs, an aperture ratio decreases and a screen looks dark.
Against this background, a technology called overdrive has been developed which improves response characteristics by making innovations in a driving method. In this technology, when a black display changes into an intermediate tone display, a change in the states of liquid crystal molecules is accelerated by changing a voltage in three steps in the following sequence: a black display voltage (low voltage), a white display voltage (high voltage), and an intermediate tone display voltage (intermediate voltage).
However, in overdrive, a voltage supplied to a data bus line needs to be changed in three steps in the following sequence: a black display voltage, a white display voltage, and an intermediate tone display voltage. Thus, overdrive has the disadvantage that a driving circuit becomes complex. Furthermore, overdrive enables response time to be shortened when a black display is changed into an intermediate tone display, but does not enable the response time to be shortened when a black display is changed into a white display because a voltage higher than that in a white display cannot be applied.
Japanese Unexamined Patent Publication No. 2003-172915 discloses that when a black display is changed into a white display, a voltage higher than a white display voltage (maximum tone voltage) is applied. However, in this case, a display voltage also needs to be changed in three steps. Moreover, it becomes necessary to form TFTs having high breakdown voltages or a driver for driving. Thus, there also arises the problem that a change of design or a change of a process becomes necessary.